Not Applicable
Not Applicable
1. Field of the Invention
This invention is related to a bipolar plate for a fuel cell, in particular, a bipolar plate with improved effect of water management, and most particularly, a bipolar plate with better water management effect utilized in a proton exchange membrane fuel cell. The bipolar plate provided by this invention involves simplified construction, therefore, the cast for manufacturing of the fuel cell is reduced and mass production of the fuel cell becomes feasible. Furthermore, the bipolar plate of this invention ensures that the fuel cell can be effectively operated under high efficiency.
2. Description of the Related Art
With the rapid growth of human civilization, the consumption of traditional energy sources, such as coal, oil and natural gas, increases rapidly. With the rapid growth of human civilization, the consumption of traditional energy sources, such as coal, oil and natural gas, increases rapidly. This has resulted in a serious pollution to the global environment, as well as, various environmental problems such as global warming and acid rain. It is now recognized that the natural energy resources are limited and, if the present rate of consumption continues, all existing energy sources will be exhausted in the very future. Accordingly, many developed countries are dedicated to the research and development of new and replaceable energy sources. The fuel cell is one of the most important and reasonably priced choices for an energy source. Compared with the traditional internal combustion engine, the fuel cell has many advantages such as high energy transformation efficiency, clean exhaust, low noise, and no consumption of gasoline.
In brief, fuel cell is an electrical power generation device by the electrochemical reaction of hydrogen and oxygen. In basic term, the reaction is a reverse reaction of the electrolysis of water, to transfer the chemical energy into the electrical energy. The basic construction of a fuel cell, for example a proton exchange membrane fuel cell, comprises a plurality of cell units. The construction of the cell unit is generally illustrated in FIG. 1. It comprises a proton exchange membrane (PEM) 10 at the middle with the two sides thereof provided with a layer of catalyst 12 and each of the two outsides of the catalyst 12 being further provided with a gas diffusion layer (GDL) 14. An anode plate 16 and a cathode plate 18 are further provided at the outermost sides adjacent to the GDL 14. After tightly combining all the above elements together, a cell unit is formed. For practical application of the fuel cell, a plurality of the above cell units are stacked and serially connected to provide sufficient power, as illustrated in FIG. 2. Therefore, two adjacent cell units can share a common polar plate 20, as illustrated in FIG. 3, which serves as the anode and the cathode for the two adjacent cell units, respectively. Accordingly, such a polar plate 20 is usually referred to a bipolar plate. Generally, as illustrated in FIG. 3, the two sides of the bipolar plate 20 are provided with many groove type gas channels 22 for delivering the gases for reaction, such as hydrogen and air (to provide oxygen), as well as moving the reactants, such as water droplet or vapor, out of the bipolar plate 20.
It should be noted that the gas within the bipolar plate 20 of the fuel cell must contain substantial moisture so that the ion from the chemical reaction can be carried by the moisture to pass through the PEM 10 for proton conduction. If the amount of moisture contained in the gas is less than the required amount, the PEM will be dehydrated, which will increase the electrical resistance and lower the voltage of the fuel cell and the life of the fuel cell will be shortened. Accordingly, the fuel cell might include a humidifier to increase the moisture content in the gas to overcome the above problem. However, if the amount of moisture contained in the gas is over the required amount, the gas channels 22 of the bipolar plate 20 for transporting gas might be clogged, as shown in FIG. 4. As a result, the gas cannot keep transporting, and the chemical reaction of the fuel cell will be terminated. Thus, excessive amount of moisture contained in the gas will also impair the performance of the fuel cell. In conclusion, water management of the gas in the bipolar plate is critical to the performance of the fuel cell.
One of the most effective measures to prevent the gas channels 22 of the bipolar plate 20 from being clogged by a water droplet 24, is to increase the pressure difference (xcex94P) between the inlet and outlet of the gas channels 22. When the gas channels 22 is clogged by a water droplet 24, as illustrated in FIG. 4, the gas within the gas channels 22 can no longer be transported. Therefore, the pressure at one side of the water droplet 24 will be substantially the same with the pressure at the inlet of the gas channels 22, and the pressure at the other side of the water droplet 24 will be substantially the same with the pressure at the outlet of the gas channels 22. Because such a xcex94P will be applied on both sides of the water droplet 24 when it clogs the gas channels 22, increased xcex94P between the inlet and outlet of the gas channels 22 will shatter and blow the water droplet 24 out of the gas channels 22.
The conventional measure to increase the xcex94P between the inlet and outlet of the gas channels is to decrease the diameter of the gas channels and/or increase the length of the gas channels which thereby is in a slender serpentine formation, as shown in FIG. 5. The reason for providing such a slender serpentine formation is that the wall of the gas channels will introduce frictional resistance to the gas when it transports in the gas channels. Thus, a larger xcex94P between the inlet and outlet of the gas channels can be obtained. Nevertheless, this conventional measure needs to manufacture the gas channels into a single elongated serpentine formation with very small diameter. Such a formation will significantly increase the difficulty and costs for producing the bipolar plate 20. Additionally, such a slender serpentine gas channel will cause turbulence or other unexpected disturbance to the transportation of gas and thus, the chemical reaction and the efficiency of the fuel cell will be affected.
The primary object of this invention is to overcome the disadvantages of the conventional technique and to provide a bipolar plate for a fuel cell. The bipolar plate has a plurality of groove type gas channels parallel with one another. Each of the gas channels has a cross section defined by the walls thereof, and each of the gas channels has an opening. In the vicinity of each of the openings is provided with a reducing device to reduce the area of the cross section. By reducing the area of the cross section of the groove type gas channels, this invention can increase the pressure difference between the inlet and outlet of the gas channels. Thus, the gas channels of the bipolar plate will not be clogged by water droplets and the fuel cell can be operated with high efficiency.
The structures and characteristics of this invention can be realized by referring to the appended drawings and explanations of the preferred embodiments.